1. Field of the Invention
The present invention relates to an acoustic attenuator, to an apparatus for well logging and to a method of logging a well. In another aspect, the present invention relates to an acoustic attenuator, an acoustic well tool apparatus having such an attenuator, and to a method of performing acoustic investigations in a well with such a well tool. In still yet another aspect, the present invention relates to an acoustic attenuator for attenuating sonic waves travelling along a well tool, to an acoustic well tool apparatus having attenuation of the sonic waves travelling along the well tool, and to a method of attenuating acoustic signals using such a well tool.
2. Description of the Related Art
Hydrocarbon exploration and data gathering continues even after the drilling of a well, generally by utilizing wireline logs that are obtained to reveal certain physical characteristics of the formation. The formation porosity and water saturation may be derived by measuring formation resistivity, neutron and gamma ray values.
Formation porosity may also be obtained by taking sonic logs utilizing a sonic well logging tool. Sonic logs are also taken to obtain the speed of propagation of sound in the rock, and to obtain information on the bulk elastic properties of the formation.
Such information that may be obtained from sonic logs is useful in a variety of applications, including well to well correlation, determining porosity, determining the mechanical or elastic parameters of the rock and hence an indication of lithology, detecting overpressured formation zones, and enabling the conversion of a seismic time trace to a depth trace from information acquired on the speed of sound in the formations.
In addition to being able to obtain resistivity, neutron and gamma ray investigations using wireline tools, oilfield technology developed in the late 1980's produced alternatives to wireline logs, mainly, the resistivity, the neutron and the gamma ray density measurements can now be performed on the drill string while drilling, i.e. "measuring while drilling" or "MWD".
However, similar developments of a "while drilling" alternative to the fourth most commonly used log, the sonic log, has not been as successful as with the "while drilling" alternative for taking resistivity, neutron and the gamma ray density logs.
With a sonic log, an acoustic signal is generated at a transmitting transducer, which signal propagates through the formation to at least one receiving transducer and the time of first signal arrival is detected. As the time of transmission is known, and the time of the first arrival of acoustic energy at the receiver after having passed through the formation is measured, the time of propagation of the signal through the formation which is referred to as interval transit time .DELTA.t can now be calculated. This interval transit time may then be used in the Wyllie time-average equation, .DELTA.t=.DELTA.t.sub.solid (1-.phi.)+.DELTA.t.sub.fluid (.phi.) , to obtain porosity .phi.. .DELTA.t.sub.solid and .DELTA.t.sub.fluid are known from predetermined knowledge of the speed of propagation of sound in various rocks and fluids and by knowing the types of rock and fluid in which the investigation is being made. As an alternative to the Wyllie time average relationship, the more recent "Raymer, Hunt, Gardner" relationship may also be utilized.
While it is desired that signals generated by the sonic tool transmitter travel to the sonic tool receiver via the formation, it is also possible for such signals to travel from the transmitter to the receiver via the tool body itself. It is desirable that the detected signal be virtually free of energy launched by the transmitter into the tool body which is then propagated along the tool body to the position of the receiver.
The velocity of sound through liquids of the type typically found in a well bore is on the order of 5,000 feet per second. On the other hand, the velocity of sound through earth formations is in the range of about 5,000 to about 25,000 feet per second. By way of comparison, metals can have sonic propagation velocities ranging between 13,000 and 20,000 feet per second.
Since the speed of propagation of sound in the tool body, which is normally steel, can in some instances be much higher than that of the formation rock, the tool signal arrival in those cases invariably occurs before the formation arrival. As a sonic tool merely records signals as they are obtained, the tool has no way of distinguishing whether a signal has traversed the formation or the tool body. Thus, such a first arriving signal traversing the tool body may be confused as the first arriving signal traversing the formation.
Fortunately, various techniques have been developed in the wireline logging industry for attenuating and/or slowing down the tool propagated signal so that formation arrival may be detected without much interference from the tool propagated signal. In such wireline investigations, the tool is not required to function as a load bearing member so that it has been possible to form an array of staggered openings through the width of the sidewall of the tool's housing. These openings serve to lengthen the total path length that a housing propagated acoustic signal must follow so that the signal across an extremely broad range of frequencies is not only delayed in its transit of the array of holes, but is also attenuated as a result of the increased path length and the signal scattering caused by the openings.
U.S. Pat. No. 3,381,267, issued Apr. 30, 1968 to Cubberly, Jr. et al. discloses a well logging tool having a rigid housing with a large number of reversed paths and interface surfaces to provide an extremely tortuous path for attenuation of sonic energy that would otherwise travel longitudinally along the housing between the transmitter and receiver.
U.S. Pat. No. 3,493,921, issued Feb. 3, 1970, to Johns discloses a sonic well logging tool which utilizes between the transmitter and receiver, a configuration of collapsed bellows, which configuration is stated as achieving desired characteristics as to structural integrity, sonic delay and sonic attenuation.
U.S. Pat. No. 5,036,945, issued Aug. 6, 1991, to Hoyle et al. discloses a sonic well tool having a first and second attenuation and delay apparatus for attenuating and delaying the signal traversing the tool body. The first attenuation and delay apparatus includes interleaved rubber and metal like washers for attenuating compressional and flexural waves propagating along the body, and further includes a bellows section having a corrugated shape and a thin traverse dimension. The second attenuation and delay apparatus includes mass loading rings surrounding the housing of the well tool, and also includes a bellows section having a corrugated shape and a thin traverse dimension.
U.S. Pat. No. 5,229,553, issued Jul. 20, 1993 to Lester et al. discloses an acoustic isolator for use with a well logging tool having transducers in a first and third tool segment, which are to be acoustically isolated from receivers in a second and fourth tool segment. The acoustic isolator consists of vertebrate links composed of spools, encased by resilient boots, which spools are arranged end-to-end in tandem configuration. A plurality of split shells interconnect the spools by externally gripping the boots covering the end portions of the respective adjacent spools.
The expedient of providing openings or cuts that extend through the side wall thickness of the sonic well tool is clearly unsatisfactory for a sonic investigation performed from a drilling string or drill collar. In the measuring while drilling ("MWD") environment, the sonic tool is incorporated into the drill collar and must be able to withstand the immense forces and accelerations encountered during the drilling of the well. Large numbers of perforations through the side wall of the drill collar would weaken the collar so that it would no longer be able to withstand normal wear and tear of drilling. Additionally, the fluid isolation between the inside of the drill collar and its exterior would be lost.
Openings or cuts extending through only a fraction of the width of the drill collar have not been thought to be effective since the remaining portion of the drill collar has previously been thought to provide a "straight through" path for the unattenuated propagation of the acoustic signal.
Additionally, the prior art expedient of "convoluting" the sidewall of the sonic tool so that the tool has a uniformly thick, yet tortuous longitudinal cross-section is equally unsatisfactory in that such shapes either are too weak or require too large a portion of the limited diameter of the tool.
E.P. No. 0 375 549, published Jun. 27, 1990 discloses a method and apparatus for performing acoustic investigations in a borehole. The sonic tool includes a plurality of axially periodic, substantially circumferentially continuous sections of the drill collar (such as grooves or ridges in the form of circumferential rings or helical threads) with acoustic propagation characteristics different from the drill collar to attenuate and delay signals traversing the sonic tool. E. P. No. 0 375 549 also discloses "portions" formed in the surface of the drill collar at random locations, which may be filled with epoxy, fiberglass or some other material having significant resistance to the abrasion expected through contact with the borehole wall during the drilling process. While E. P. No. 0 375 549 does provide for some attenuation of the signals traversing the sonic tool body, greater attenuation is still desired.
While these prior art inventions attempt to address the problem of sound waves traversing the sonic tool, they each either suffer from one or more limitations and/or do not provide the desired level of attenuation.
Therefore, a need exists in the art for an improved sonic well logging tool.
There is another need in the art for an improved attenuation apparatus.
There is even another need in the art for an improved method of sonic well logging.
There is still another need in the art for an improved well logging tool in which sonic signals traversing the body of the sonic well tool do not cause undue interference with the desired sonic signals traversing the formation.
There is yet another need in art for a method of sonic well logging in which sonic signals traversing the body are attenuated sufficiently so that they do not cause undue interference with the desired sonic signals traversing the formation.
There is even yet another need in the art for an improved acoustic logging while drilling ("AWD") tool.
There is still yet another need in the art for an improved method of performing acoustic logging while drilling.
These and other needs in the art will become evident to those of skill in the art upon review of this application.